The organic light-emitting diodes OLED technology has stimulated intensive research activities across all disciplines. Currently, great efforts in materials research have been focused on novel materials for full-color flexible displays. Full-color displays require three basic colors, red, green and blue, and flexible substrates require low temperature and easy processing of the organic materials. OLED devices show great promise in meeting both requirements, since the emission color can be tailored by modulation of the chemical structures and the solution processing permits for micro-patterning of the fine multicolor pixels via inkjet printing technique (Yang, Y., et al., J. Mater. Sci.: Mater. Elecron., 2000, 11, 89). However, processable, stable, and efficient blue light-emitting organic materials are still highly desirable to meet the challenge. Blue light requires wide energy band. With blue light-emitting compounds as primary materials, it is possible to produce other colors by a downhill energy transfer process. For instance, a green or red electroluminescent EL emission can be obtained by doping a blue EL host material with a small amount of green or red luminescent material.
General EL Device Architecture
The present invention can be employed in most organic EL device configurations. These include very simple structures comprising a single anode and cathode to more complex devices, such as passive matrix displays comprised of orthogonal arrays of anodes and cathodes to form pixels, and active-matrix displays where each pixel is controlled independently, for example, with thin film transistors (TFTs).
The selection of benzimidazole/amine-based compounds of the present invention allows for a high-efficiency single-layer blue-emitting EL device. A Single-layer device is the preferred EL device structure of this invention, which is simpler and less costly to fabricate than multi-layer device structures. To our knowledge, no single-layer blue-emitting EL device has been previously reported that exhibit sufficient efficiency.
Outside of the single-layer device configuration, there are numerous configurations of the organic layers wherein the present invention can be successfully practiced. A typical structure is shown in FIG. 1 and includes a substrate 101, an anode 103, a hole-injecting layer 105, a hole-transporting layer 107, a light-emitting layer 109, an electron-transporting layer 111, and a cathode 113. These layers are described in detail below. FIG. 1 is for illustration only and the individual layer thickness is not scaled according to the actual thickness. Note that the substrate can alternatively be located adjacent to the cathode, or the substrate can actually constitute the anode or cathode. The organic layers between the anode and cathode are conveniently referred to as the organic EL element. Also, the total combined thickness of the organic layers is preferably less than 500 nm.
The anode and cathode of the OLED are connected to a voltage/current source 250 through electrical conductors 260. The OLED is operated by applying a potential between the anode and cathode such that the anode is at a more positive potential than the cathode. Holes are injected into the organic EL element from the anode and electrons are injected into the organic EL element at the anode. Enhanced device stability can sometimes be achieved when the OLED is operated in an AC mode where, for some time period in the cycle, the potential bias is reversed and no current flows. An example of an AC driven OLED is described in U.S. Pat. No. 5,552,678.
Substrate
The OLED device of this invention is typically provided over a supporting substrate 101 where either the cathode or anode can be in contact with the substrate. The electrode in contact with the substrate is conveniently referred to as the bottom electrode. Conventionally, the bottom electrode is the anode, but this invention is not limited to that configuration. The substrate can either be light transmissive or opaque, depending on the intended direction of light emission. The light transmissive property is desirable for viewing the EL emission through the substrate. Transparent glass or plastic is commonly employed in such cases. The substrate can be a complex structure comprising multiple layers of materials. This is typically the case for active matrix substrates wherein TFTs are provided below the EL layers. It is still necessary that the substrate, at least in the emissive pixilated areas, be comprised of largely transparent materials such as glass or polymers. For applications where the EL emission is viewed through the top electrode, the transmissive characteristic of the bottom support is immaterial, and therefore can be light transmissive, light absorbing or light reflective. Substrates for use in this case include, but are not limited to, glass, plastic, semiconductor materials, silicon, ceramics, and circuit board materials. Again, the substrate can be a complex structure comprising multiple layers of materials such as found in active matrix TFT designs. Of course it is necessary to provide in these device configurations a light-transparent top electrode.
Anode
When EL emission is viewed through anode 103, the anode should be transparent or substantially transparent to the emission of interest. Common transparent anode materials used in this invention are indium-tin oxide (ITO), indium-zinc oxide (IZO) and tin oxide, but other metal oxides can work including, but not limited to, aluminum- or indium-doped zinc oxide, magnesium-indium oxide, and nickel-tungsten oxide. In addition to these oxides, metal nitrides, such as gallium nitride, and metal selenides, such as zinc selenide, and metal sulfides, such as zinc sulfide, can be used as the anode 103. The anode can be modified with plasma-deposited fluorocarbons. For applications where EL emission is viewed only through the cathode electrode, the transmissive characteristics of anode are immaterial and any conductive material can be used, transparent, opaque or reflective. Example conductors for this application include, but are not limited to, gold, iridium, molybdenum, palladium, and platinum. Typical anode materials, transmissive or otherwise, have a work function of 4.1 eV or greater. Desired anode materials are commonly deposited by any suitable way such as evaporation, sputtering, chemical vapor deposition, or electrochemical means. Anodes can be patterned using well-known photolithographic processes. Optionally, anodes can be polished prior to application of other layers to reduce surface roughness so as to reduce shorts or enhance reflectivity.
Hole-Injection Layer (HIL)
Although not always necessary, it is often useful that a hole-injecting layer 105 be provided between anode 103 and hole-transporting layer 107. The hole-injecting material can serve to improve the film formation property of subsequent organic layers and to facilitate injection of holes into the hole-transporting layer. Suitable materials for use in the hole-injecting layer include, but are not limited to, porphyrinic compounds as described in U.S. Pat. No. 4,720,432, plasma-deposited fluorocarbon polymers, as described in U.S. Pat. No. 6,208,075, and some aromatic amines, for example, m-MTDATA (4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine). Alternative hole-injecting materials reportedly useful in organic EL devices are described in EP 0 891 121 A1 and EP 1 029 909 A1.
Hole-Transporting Layer (HTL)
The hole-transporting layer 107 of the organic EL device in general contains at least one hole-transporting compound such as an aromatic tertiary amine, where the latter is understood to be a compound containing at least one trivalent nitrogen atom that is bonded only to carbon atoms, at least one of which is a member of an aromatic ring. In one form the aromatic tertiary amine can be an arylamine, such as a monoarylamine, diarylamine, triarylamine, or a polymeric arylamine. Exemplary monomeric triarylamines are illustrated by Klupfel, et al., U.S. Pat. No. 3,180,730. Other suitable triarylamines substituted with one or more vinyl radicals or at least one active hydrogen-containing group are disclosed by Brantley, et al. in U.S. Pat. Nos. 3,567,450 and 3,658,520
Other Useful Organic Layers and Device Architecture
In some instances, the light-emitting layer 109 and the electron-transporting layer 111 can optionally be collapsed into a single layer that serves the function of supporting both light emission and electron transportation. Alternatively, the light-emitting layer 109 and the hole-transporting layer 107 can optionally be collapsed into a single layer that serves the function of supporting both light emission and hole transportation. Alternatively, layers 107, 109 and 111 can optionally be collapsed into a single layer that serves the function of supporting both light emission and hole and electron transportation. This is the preferred EL device structure of this invention and is referred to as “single-layer” device. In contrast to multi-layer device structures, fabrication of a single-layer device is simpler and less costly.
To our knowledge, no single-layer blue-emitting EL device has been previously reported that exhibit sufficient efficiency. The selection of benzimidazole/amine-based compounds of the present invention allows for a high-efficiency single-layer blue-emitting EL device.
It also known in the art that emitting dopants can be added to the hole-transporting layer, which can serve as a host. Multiple dopants can be added to one or more layers in order to produce a white-emitting EL device, for example, by combining blue- and yellow-emitting materials, cyan- and red-emitting materials, or red-, green-, and blue-emitting materials. White-emitting devices are described, for example, in EP 1 187 235, EP 1 182 244, U.S. patent application Publication 2002/0025419 A1, and U.S. Pat. Nos. 5,683,823, 5,503,910, 5,405,709, and 5,283,182.
Additional layers such as electron or hole-blocking layers as taught in the art can be employed in devices of this invention, Hole-blocking layers are commonly used to improve efficiency of phosphorescent emitter devices, for example, as in U.S. patent application Publication 2002/0015859 A1.
This invention can be used in so-called stacked device architecture, for example, as taught in U.S. Pat. Nos. 5,703,436 and 6,337,492.